Producing ethylene by methane dehydrogenation is an energy-intensive reaction. Since the reaction is endothermic and reaction temperatures greater than 800° C. are generally required to achieve practical methane conversion levels, a significant amount of heat is required to maintain the reaction. Generating this heat and transferring it to the methane is a significant cost, and can introduce inefficiencies into the process. In order to overcome some of these difficulties, there has been considerable effort directed toward methane conversion via catalytic oxidative coupling reactions.
One process for producing ethylene from methane by catalytic oxidative coupling is disclosed in Synthesis of Ethylene via Oxidative Coupling of Methane, G. E. Keller and M. H. Bhasin, Journal of Catalysis 73, 9-19 (1982). Although an appreciable selectivity to ethylene was observed (to a maximum of about 50%), conversion was relatively low. In order to overcome the methane-ethylene separation difficulties resulting from the low methane conversion, technology has been developed for quenching the reaction product downstream of the oxidative coupling reactor, and then separating ethylene from the unreacted methane.
One process, disclosed in Enhanced C2 Yields from Methane Oxidative Coupling by Means of a Separative Chemical Reactor, A. E. Tonkovich, R. W. Carr, R. Aris, Science 262, 221-223, 1993, includes a simulated countercurrent moving-bed chromatographic reactor, and achieves 65% methane conversion and 80% selectivity to C2 hydrocarbons. The reactor is configured in four sections, with each section comprising (i) a catalytic reactor containing Sm2O3 catalyst and (ii) an adsorbent column located downstream of the catalytic reactor. Methane and oxygen react via catalytic oxidative coupling in the reactor at a temperature in the range of about 900° K to 1100° K, and then ethylene is separated from unreacted methane in the sorption column. In order to maintain sufficient selectivity for ethylene sorption, the reactor's product is quenched to a temperature of 373° K in the sorption column. In another process, disclosed in Methane to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor-Separator, Y. Jiang, I. V. Yentekakis, C. G. Vayenas, Science 264, 1563-1566, 1994, gas recycle is utilized to further increase methane conversion, but an even lower quench temperature (30° C.) is used during ethylene sorption.
Although the disclosed moving-bed and gas-recycle processes improve conversion, the quenching is energy intensive, and further improvements are desired. Further improvements are particularly desired in converting alkanes such as methane into C2+ olefins such as ethylene and propylene, particularly with increasing selectivity to ethylene production.